Two-way signaling system



moms@ Feb. 15, 1938.

G, ABRAHAM TWO-WAY 'SIGNALING SYSTEM Filed Oct. 29, 1936 r wpressozr VI'SI',

INVENTOR BY L, 6. @wa/uw@ ATTORNEY Patented Feb. 15, 1938 PATENT orificeTWO-WAY SIGNALING SYSTEM Leonard Gladstone Abraham, Madison, N. J.,assignor to the American Telephone and Telegraph Company, a corporationof New York Application October 29, 1936, Serial No. 108,279

Claims.

'Ihis invention relates to two-way transmission systems and moreparticularly to two-way tele:- phone systems including a long four-Wiresection havin-g separate paths for transmission in oppo- 5 sitedireptions.

In a four-wire circuit such as that contemplated there arise certainproblems. Each end of the circuit may be connected to an extendingci-rcuit, which is often a two-wire line. It is the usual practice toconnect the end of the fourwire circuit and the extending two-wire linetogether through a hybrid coil or the equivalent and to provide anarticial network designed to balance the impedance of the two-wire line.'Ihis balance is at the best an approximation, and theinevitable lack ofperfect balance between the impedance of the two-wire line and theimpedance of the artificial network results in the reflection of energy,whichis normally transmitted in one direction over one side of thefourwire circuit, back over the oppositely `directed path. As a resultof this reflection there arise, as is Well understood in the art,various transmission problems, including those of echoes and singing.For the purpose of preventing singing and the objectionable transmissionof echoes it is the practice to utilize certain transmission controlapparatus usually responsive tol energy such as that representing thevoice in the case of a telephone system. The use of such apparatus,however, leads to further problems, such as those of unduly difficultbreaking and mutual lockout.

In the applicants Patent No. 2,060,008 issued 35 November 10, 1936(application Serial No. 740,-

518) there' is disclosed an arrangement of transmission controlapparatus which for many purposes solves or satisfactorily approaches asolution of the lproblems indicated hereinbefore. With that arrangement,however, there may be an appreciable chance of sustained singing, aswill be indicated hereinafter. The principal object of the presentinvention is to eliminate or substantially eliminate this possibility ofsinging, which possibility may be tolerable in many instances of the useof such a system, but quite intolerable in the exceptional case.

The present invention contemplates, as indicated hereinbeiore, afour-wire circuit, vwhich many diiferent types of two-Way circuit. Forinf 55 stance, it is applicable to a four-Wire circuit which may be ofgreat length, and transmission control is extended at one end by a landtwo-wire line and at the other end by a submarine cable circuit. Again,the four-wire circuit to the terminals of which the transmission controlapparatus is directly applied may include sections inter- 5 mediate toWhich are hybrid coils or the equivalents, no intermediate transmissioncontrol apparatus being applied. Also it will be clear to those skilledin the art that the oppositely directed paths of the four-Wire circuitmay include a radio link or radio links.

In general, the applicants terminal arrangements include at or near eachterminal. of the four-wire circuit a vodas, an echo suppressor and adisabler. As is well understood in the art, the term vodas is nowgenerally applied to the voice-operated device anti-singing andrepresents the words describing such apparatus. Briefly, with referenceto the application to the present invention, each one-way path of the 20four-wire circuit is normally disabled or includes an appreciabletransmisson loss at a point near its receiving end, such disability orloss having the primary purpose of preventing the completion of asinging path. When the voice energy, for example, reaches the receivingend of the one-way path, the vodas operates to remove the disability orother loss, and thus to permit the completion of the transmission. Theecho suppressor, as applied in the present case, serves to disable. theoppositely directed transmitting path under certain conditionssimultaneously with the clearing of the receiving path. The disabler isa device which permits outgoing voice energy, for` example, which passesthe point in the transmitting path at which the echo sup-- pressor maytake effect to prevent the subsequent interruption of the outgoing wavesin response to later arrivmg energy in the incoming path. Such anarrangement of terminal transmission control apparatus is disclosed andclaimed in the applicants Patent No. 2,060,008, issued Novem-` ber l0,1936, referred to hereinbefore.

As Will appear more clearly hereinafter, the arrangement just described,without more, may permit sustained singingif the two subscribersconnected to the ends of the transmission system begin to speak atapproximately the same time. In general, it is the outstanding featureof the p-resent invention that there'is introduced into the operation ofthe disabler a definite delay, Which by itself, or in cooperation withcertain other arrangements to be discussed hereinafter, serves toeliminate, or'reduce as far as 55A is feasible, the chance of thesinging condition which may be so produced.

The following description of one desirable embodiment of the inventionis to be read with reference to the accompanying drawing, which showsdiagrammatically and in part schematically the four-wire circuit,including the application at or near each terminal thereof of theapplicants transmission control apparatus with the above indicated delaysuitably provided for.

With reference to the drawing there is shown a four-wire circuitextending between two widely separated stations W and E. This four-wirecircuit is extended at its left end by a two-wire line L, whichfurnishes the connection to the station of the west subscriber labeledSW. At the right end the four-wire circuit is extended by a twowire lineL which may form the circuit of a submarine cable SC. The connectionsbetween the four-wire circuit and the lines L and L' are through hybridcoils H and H, respectively, the artificial networks N and N furnishingan approximate balance with the lines L and L', respectively.

The four-wire circuit extending between stations W and E comprises thetwo-wire path LL adapted for transmission from west to east, forexample, and the two-wire path LL adapted for transmission from east towest. The inclusion of suitable amplifying elements is indicated at Aand A. It will be understood from the statements made hereinbefore thatthe paths LL and LL may not be continuous, but may involve the inclusionof different links, including one or more radio links. It is to beunderstood, furthermore, that for the best results from the applicantspresent arrangement the system is one which does not involvetransmission control apparatus intermediate to stations W and E.

At station W the transmission control apparatus comprises the vodas V,including relay R1, the echo suppressor S, including relay R2, and thedisabler D, including the relay R5. Correspondingly, the apparatus atstation E comprises the vodas V with relay R'1, the suppressor S withrelay Rz and the disabler D', including relay Rs. Thus it appears in thearrangement specifically disclosed that the system is symmetrical withrespect to the transmission control apparatus at the two terminalstations of the four-wire circuit. While such arrangement is usually tobe preferred, it will be understood from an examination of the appendedclaimsV that the invention is not necessarily limited in its scope tothis symmetrical arrangement.

It is believed that the arrangement and operation of the transmissioncontrol apparatus of the present invention and the nature of theimprovement now made over the applicants system disclosed in Patent No;2,060,008 issued November 10, 1936 will be best understood from thefollowing detailed description, which is based on the operation of theapparatus.

When the subscriber at SW talks, the energy representing his voicetravels over line L and divides in hybrid coil H, the useful portiontraveling on over path LL. This energy is amplified at A and will reachpoint d, unless it is blocked by reason of the fact that the suppressorrelay R2 is operated. At point d, a small portion of the energytravelling from west to east in path LL is diverted to operate theamplifier-detector AD2. Connected to the output of theamplifier-detector is a delay circuit DC, which, in the specicdiclosureV now made, would be a direct current delay circuit. To theoutput of the circuit DC is connected the winding of the disabler relayR5. It will be noted that the operation of relay R5 breaks a circuitassociated with the output of an amplier-detector AD1, which is operatedby energy diverted from the path LL' at point b. Accordingly, if relayR5 operates before the operation of the suppressor relay R2, there willbe no change in the transmission loss introduced in Vpath LL at point c.In other Words, if energy traveling from west to east passes point c andcauses the operation of the disabler relay R5 Vbefore energy travelingin the opposite direction in path LL causes the operation of thesuppressor relay R2, then the subscriber at SW has control of thecircuit and the energy representing his voice will travel on to stationE. However, it is to be borne in mind that the circuit DC introduces aconsiderable delay in the operation of relay R5, which feature will beconsidered in greater detail hereinafter.

Once the energy representing the west subscriber passes the point d, itwill travel on to the east end of the four-Way circuit, regardless ofthe presence or absence of energy traveling in the opposite directionover path LL. This feature of the applicants arrangement will be readilyunderstood when it is considered that a portion of the West-east energyis diverted at point b to operate amplifier-detector AD1 and relay Ri.It may be that energy traveling from east to west in path LL willprevent the operation of the suppressor relay Rz in response to theoperation of amplifier-detector ADi, but this disabling operation has noeffect on the operation of relay Ri, which is the relay of the vodas V.For the purpose of suppressing singing, the path LL is normallydisabled, or has an appreciable transmission loss, at point a. Asspecifically disclosed, this loss is introduced by the shunt circuitincluding the resistance ri. When the vodas relay Ri operates inresponse to the incoming signal energy in path LL, this shunt normallyapplied at point a is removed and the signal energy is free to pass onto the extending two-Wire circuit L.

Similarly when the subscriber connected to the right (east) end of thesystem begins to speak, the energy representing his speech travels overpath LL and passes point c', unless the echo suppressor relay Rz isoperated. It is understood, of course, that if this relay is operated,there is closed a shunt including a resistance z, which circuitintroduces an appreciable loss at point c'. After passing point c', theeast-west energy operates amplier-detector ADz, and after the perioddetermined by the delay circuit DC', operates the disabler relay R5,unless such operation is prevented. After passing the east end of pathLL', the east-west energy will pass on without fail (provided only it isof sufficient magnitude) to the west end of the circuit, operatingamplifierdetector AD1 and the vodas relay R1, which removes thedisability at point a. This disability, as specifically disclosed, isproduced by the application of the shunt including resistance r1.

It will be understood by those skilled in the art that various factors,including the relative operating time of the various transmissioncontrol devices, enter into the satisfactory operation of this system.It will be understood that without a suitably timed delay in theoperation of the disabler, it is possible for a condition of sustainedsinging to be set up-if the subscribers at the ends of the circuithappen to start speaking at about the same time and if the circuitcourse, remove the disability in the rec'eivingseci tion at station E,while the oppositely directed energy operated the disabler at E,preventing the operation of the echo suppressor at E, and, of course,passed on to remove the disability nor- /mally applied in the receivingsection at W.

While such a condition is not likely to occur at anygiven time, it islikely to occur eventually, and it is highly important for the properoperation of some transmission circuits that this possibility beeliminated or rendered quite remote.

Accordingly, the applicant proposes to introduce into the operation ofthe disabler D a delay which is equal to the one-way delay of the four-`wire circuit; that is, for example, the delay of path LL between pointsd and b. Likewise it is proposed to introduce an equal delay in theIdisabler D at the opposite end of the circuit which will, of course,equal the delay between W and E, or from point d to b. It will be notedthat the output of amplifier-detector ADi includes not only the windingof relay R2 but also the windings of relays R3 and R4. These relays aredesigned to break the disabler circuit, to prevent the operation of thedisabler relay R5 after amplifier-detector AD; has operated. 'I'hepurpose of this arrangement, including the delay in the operation of thedisabler, is to give control of the circuit to the subscriber who firstbegins to speak. If the west subscriber begins before the eastsubscriber, he will, of course, get through at point c, and the energydiverted at point d will operate the disabler relay R5, after a delaywhich is completed before the later originating energy from E canoperate the suppressor relay R2 at W, or

the relays R3 and R4, which will affect the operation of the disablerrelay R5. Thus, while the west-east energy upon reaching E will causethe removal of the disability at point a', the disability at W appliedat point c remains to prevent the completion of a singing path.

It is necessary to a complete understanding of the present invention andthe features of the applicants circuit to consider the various relaysensitivities and the hangover periods involved. In accordance with theinvention in the form specifically disclosed, the Zero level sensitivityof the disabler relay R5 should be slightly lower than the sensitivityof the suppressor and vodas relays Rz and R1, respectively, at theopposite end of the four-Wire circuit, under all conditions of circuitnet loss. Correspondingly, the sensitivity of relay Rs at the east endshould be slightly lower than the sensitivity of relays R2 and R1 at theWest end of the circuit. The hangover of the disabler relay R5 atstation W-and that of the vodas relay Ri-should not exceed the hangoverof the suppressor S, which will be largely determined by the hangover ofrelay R2.

With greater refinement, it appears that the desirable sensitivities forthe suppressor relay R2 and the vodas relay R1 are sensitivities whichare equal; then if the transmission control apparatus applied to thewhole four-wire circuit is symmetrical, the sensitivity of the disablerrelay R5 should be somewhat lower than the sensitivities of R2 and R1.In other words, the sensitivities should be so related that the zerolevel sensitivity of the disabler relay will never become greater thanthe zero level sensitivity of the vodas relay, which latter sensitivityin turn should never be greater than the Zero level sensitivity of thesuppressor relay.

The hangover of the suppressor relay (Rz, for example) should bedetermined from the maximum delay expected in any echo, with referenceto the path between the input of the suppressor and the point at whichthe disability or increased transmission loss is applied. The hangoveroi the vodas relay (R1, for example) should be determined by the speechcharacteristics and, as

indicated hereinbefore, should not be greater'4 than the hangover of thesuppressor relay.

The arrangement disclosed with the definite delay of the disablingaction and the sensitivity and hangover relations indicated, appears togive an operation which is quite satisfactory from the standpoint ofsinging suppression, and reasonably satisfactory with respect to othercon-` siderations, such as breaking and lockout.

While the invention has been disclosed specifically in one desirableembodiment for the purpose of illustration, itis to be understood thatthe scope of the invention is to be determined from the appended claims.

What is claimed is:

l. In a two-way system for the transmission of energy, comprising twoseparated stations connected by two paths adapted for transmission inopposite directions, the method of transmission control which consistsin normally introducing an appreciable loss in each path near itsreceiving end, causing the energy approaching the receiving end ofeither path to remove said loss therefrom independently of the presenceor absence of energy in the opposite path, applying a portion of saidrst mentioned energy tointroduce an appreciable loss in the oppositetransmitting path, applying a portion of the energy traveling overeither transmitting path to prevent the subsequent introduction of lossin said path, and introducing a delay in said preventive actionsubstantially equal to the one-way delay between the stations.

2. In a two-way telephone system, two separated stations, two pathsconnecting said stations and adapted for transmission in oppositedirections, means associated with the receiving section of each of saidpaths for normally in'- troducing an appreciable loss in said section,means responsive to voice waves reaching the receiving section of eitherof said paths for removing the loss normally introduced thereinindependently of the presence or absence of voice waves in the oppositepath, means responsive to voice waves reaching the receiving section ofeither of said paths for closing a circuit designed to introduce anappreciable loss in the transmitting section of the opposite path, andmeans responsive to earlier arriving voice waves in said transmittingsection for opening said circuit but only after a delay approximatelyequal to the' one-way delay of the system between said stations.

3. In a two-way telephone system, two separated stations, two pathsconnecting said stations and adapted for transmission in oppositedirections, and corresponding sets of transmission control apparatuslocated one at each of said stations, each of said sets comprising avodas for controlling the receiving path independently of the presenceor absence of voice waves in the opposite transmitting path, an echosuppressor for introducing an appreciable loss in the transmitting pathin response to incoming voice waves, and a disabler for disabling theassociated echo suppressor inresponse to outgoing voice waves whichsucceed in passing the same, said disabler including means for delayingits action for a period of time substantially equal to the one-way delayof the system between said stations.

4. In a two-way telephone system, two separated stations, two pathsconnecting said stations and adapted for transmission in oppositedirections, and sets of transmission control apparatus located one ateach of said stations, one of said sets comprising a vodas forcontrolling the receiving path independently of the presence or absenceof voice waves in the opposite transmitting path, an echo suppressor forintroducing an appreciable loss in the transmitting path in response toincoming voice Waves, and a disabler for disabling the associated echosuppressor in response to outgoing voice Waves which succeed in passingthe same, said disabler including means for delaying its action for aperiod of time substantially equal to the one-way delay of the systembetween said stations.

5. In a two-way telephone system, two separated stations, two pathsconnecting said stations and adapted for transmission in Oppositedirections, and sets of transmission control apparatus located one ateach of said stations, each of said sets comprising a Vodas forcontrolling the receiving path independently of the presence or absenceof voice waves in the opposite transmitting path, an echo suppressor forintroducing an appreciable loss in the transmitting path in response toincoming voice waves, and a disabler for disabling the associated echosuppressor in response to outgoing voice waves which pass saidsuppressor, said disabler including means for delaying its action for aperiod of time substantially equal to the one-Way delay of the systembetween said stations, the sensitivity of the disabler being lower thanthe sensitivity of the vodas or the echo suppressor, the hangover of thedisabler apparatus not exceeding the hangover of the echo suppressor.

LEONARD GLADs'roNE ABRAHAM.

